H-1 spin diffusion and small-angle X-ray scattering (SAXS) measurements were carried out for a series of polystyrene-polyisoprene block copolymers of varying molecular weight and molecular architecture. The sizes of domains and the thickness of the interphase in the copolymers were determined from the spin diffusion measurements using a model which explicitly considers the effects bf H-1 spin-lattice relaxation. The thickness of the interphase determined by NMR was found to be constant (ca. 3 nm) for all of the copolymers. The variations in domain size and intersphere distance as a function of molecular weight were found to be in good agreement with those determined by the more established SAXS methods. However, the intersphere distances measured by NMR were found to be consistently smaller than those measured by SAXS, and this difference may be caused by simplifications in the morphological model used in the analysis of the spin diffusion curves.